The present invention is generally directed to Solid Oxide Fuel Cells (SOFC's), and more specifically to SOFC's having a circulation of a portion of the anode tail gas (i.e., exhaust).
The circulation of a portion of the anode exhaust is known in the solid oxide fuel cell field. This technique is used to provide water to the hydrocarbon fuel inlet of the SOFC to support the steam reforming of the hydrocarbon fuel. Two methods are known for conducting this anode circulation.
In the first method, a portion of the anode exhaust is simply injected back to the SOFC fuel inlet by aspiration or with a mechanical blower. In this case the composition of the anode inlet injected gas stream is similar to the anode exhaust stream. The composition of the exhaust stream is a function of the type of fuel used, the effectiveness of the reforming process, the fuel stoichiometry, and the circulation rate. The composition of the anode exhaust will typically contain water, carbon dioxide and non reacted fuel (i.e., hydrogen, carbon monoxide, and primary fuel feedstock).
In the second method, the anode exhaust is mixed with a portion of the cathode exhaust and a portion of this gas stream is injected into the SOFC fuel inlet by aspiration or with a mechanical blower. In this case, the composition of the anode inlet injected gas stream is modified from the anode exhaust. In general, the composition of the anode inlet injected gas, resulting from partial combining the anode and cathode exhausts, will have a lower fuel content (i.e., hydrogen, carbon monoxide, and primary fuel feedstock) and higher water and carbon dioxide content. Additionally, if as typical, the cathode oxygen source is from ambient air, there will be a quantity of nitrogen in the anode inlet injected gas not normally existent within the anode inlet injected gas from simple partial anode exhaust circulation.